This invention relates to a process for preparing silicon nitride powder. More particularly, it relates to a process for preparing silicon nitride powder containing .alpha.-form silicon nitride in a high proportion, being of high purity and homogeneity, and having uniform grain size and shape.
Since ceramics made of silicon nitride have high bonding strength and superior heat resistance, there has been expected that such ceramics would be applicable in strengthening materials or abrasion-resistant materials used at high temperatures. When the materials are used for such a purpose, the strength of the materials is determined by the density of the materials per se, or influenced by the size of defective hollows present inside the materials. Accordingly, it has been required for such materials to contain substantially no defective hollows and to have dense and homogeneous structures. In particular, it has been desired that the raw powdery materials be of higher quality.
The following methods are known for synthesizing the above-mentioned silicon nitride powder;
(1) Direct reaction method (in which metallic silicon powder is directly nitrogenated): EQU 3Si+2N.sub.2 .fwdarw.Si.sub.3 N.sub.4 PA0 (2) Vapor-phase reaction method (in which silicon tetrachloride or silane is reacted with ammonium in a vapor-phase, for example): EQU 3SiCl.sub.4 +4NH.sub.3 .fwdarw.Si.sub.3 N.sub.4 +12HCl PA0 (3) Silica reduction method (in which SiO obtained by reducing silica (SiO.sub.2) with substantially a stoichiometrical amount of carbon is nitrogenated; the source of silica may broadly include those having an organic group): EQU 3SiO.sub.2 +6C+2N.sub.2 .fwdarw.Si.sub.3 N.sub.4 +6CO
Some of these methods have already been brought into practical use.
Of the above methods, method (1), the direct reaction method, which is an exothermic reaction, has problems in that it requires an elaborate apparatus for controlling the heat generated. In addition, the resultant silicon nitride powder is of grain size which is so coarse as to make it difficult to produce a sufficiently fine powder. On the other hand, method (2), the vapor-phase reaction method, is suitable, for example in coating the surface of a semiconductor element because the silicon nitride is obtainable in high purity, but inevitably renders its procedures troublesome because it requires a treatment for removing hydrogen halide or the like formed by the reaction when a compound containing a halogen, such as silicon tetrachloride, is used. In addition, this method also suffers disadvantages for example the grain size and shape of the resultant silicon nitride powder are not only nonuniform, but also difficult to regulate.
Method (3), the silica reduction method, is advantageous in that the reaction is easy to carry out and, on the other hand, disadvantageous in that it results in poor yield of .alpha.-form silicon nitride which is useful as a sintering material. The present inventors, who have made a variety of studies on the methods improving for the above-mentioned silica reduction method, already found, as proposed in the Japanese Patent Publication No. 23917/1979, an improved process for preparing silicon ceramics powder, which comprises adding, as a third component, a silicon nitride (Si.sub.3 N.sub.4) or a silicon carbide (SiC) to a system comprising silica (SiO.sub.2) and carbon (C) such as silica-carbon-silicon nitride system or silica-carbon-silicon carbide system, thereby having obtained silicon ceramic powder which is of improved homogeneity and of improved uniformity in grain size and shape.